Multilayer exchange spring recording media

ABSTRACT

A multilayer exchange spring recording media consists of a magnetically hard magnetic storage layer strongly exchange coupled to a softer nucleation host. The strong exchange coupling can be through a coupling layer or direct. The hard magnetic storage layer has a strong perpendicular anisotropy. The nucleation host consists of one or more ferromagnetic coupled layers. For a multilayer nucleation host the anisotropy increases from layer to layer. The anisotropy in the softest layer of the nucleation host can be two times smaller than that of the hard magnetic storage layer. The lateral exchange between the grains is small. The nucleation host decreases the coercive field significantly while keeping the energy barrier of the hard layer almost unchanged. The coercive field of the total structure depends on one over number of layers in the nucleation host. The invention proposes a recording media that overcomes the writeability problem of perpendicular recording media.

BACKGROUND Field of Invention

This invention relates generally to magnetic recording media, and moreparticularly to thermally stable high density media.

Description of Related Art

Modern magnetic recording media reaches ever higher recording densities.Further increase of the areal density is believed to be limited by thesuperparamagnetic limit. This limit represents that as the size of themagnetic grains in the media decrease, at some grain size the thermalfluctuations at room temperature k_(B)T₃₀₀ become capable of overcomingthe energy barrier ΔE=KV, which separates the two magnetizationdirections of an isolated grain with a volume V and an uniaxialanisotropy constant K. This superparamagnetic limit, or thermalinstability, can be overcome by increasing the anisotropy K, accordingto the Stoner-Wohlfarth theory. However, such an increase also resultsin an unfavourable increase of the coercivity H_(c). As a consequence,these grains are thermally stable but can not be written with existingrecording heads.

Various improvements have been proposed to counter this thermalinstability recently, also known as the writeability problem. In U.S.Pat. No. 6,468,670 a continuous ferromagnetic overlayer was introducedto increase the Signal to Noise Ratio (SNR). U.S. Pat. Nos. 6,280,813and 6,383,668 addressed the thermal instability problem by replacing theconventional single magnetic recording layer with two ferromagneticfilms that are antiferromagnetically coupled together across anonferromagnetic spacer film, and a ferromagnetic layer that is coupledto a synthetic antiferromagnetic, respectively. This idea reduces thedemagnetizing field of the bits in the case of longitudinal magneticrecording. U.S. Pat. No. 5,583,727 proposed to overcome the problem byemploying thermally assisted recording. In the paper “FeRh/FePt exchangespring films for thermally assisted magnetic recording media” AppliedPhysics Letters, Vol. 82, Issue 17, April 2003, pp. 2859-2861. Thiele etal. suggested to lower the coercive field by the use of FePt/FeRhbilayer system. The proposed architecture included a hard layer,exchange coupled to an antiferromagnetic layer. After heating theantiferromagnetic layer across a transition temperature, it becameferromagnetic with a large magnetic moment and low magnetocrystallineanisotropy. Thus, upon crossing the transition temperature theantiferromagnetic layer acted as a magnetic soft layer that helped toreverse the hard layer.

In the paper “Composite Media for Perpendicular Magnetic Recording”.IEEE Transactions on Magnetics, Vol. 41, No. 2, February 2005. pp.537-542, R. H. Victora and X. Shen proposed magnetic multilayerstructures composed of magnetically hard and magnetically soft layers.In the model of Victora and Shen, the magnetization of the soft and thehard part of each grain remained uniform. In order to decrease thecoercive field, the exchange coupling between these layers had to bereduced with a decoupling layer. Motivated by the theoretical work. Wanget al. performed an experimental work on two layer composite media. Theresults were reported in “Composite media (dynamic tilted) media formagnetic recording”, Applied Physics Letters, Vol. 86, April 2005. pp.142504. Wang et al. concluded that a coupling layer was required incomposite media to decrease the exchange coupling between the soft andhard layer, in accordance with the theory. This was in contrast to thepaper “Exchange spring media for perpendicular recording,” AppliedPhysics Letters. Vol. 87, July 2005. pp. 12504-12507 by Suess et al.,incorporated herein by reference in its entirety, where states withinhomogeneous magnetization were formed.

The paper “Preliminary Study on (CoPtCr/NiFe)—SiO2 Hard/Soft-StackedPerpendicular Recording Media”, IEEE Transactions on Magnetics. Vol. 41.No. 10, October 2005, pp. 3136. Y. Inaba et al. considered asufficiently thin soft magnet coupled to a sufficiently thin hard magnetin order to keep the magnetization uniform and parallel in both layersduring reversal. The paper “Exchange spring recording media for arealdensities up to 10 Tbit/in²”, Journal of Magnetism and MagneticMaterials, Vol. 290-291, 2005, pp. 551-554 (available online 18 Dec.2004) by Suess et al., incorporated herein by reference in its entirety,proposed a tri-layer structure which was composed of a hard layer atbottom, a soft layer in the middle and a hard layer on top.

The paper “Exchange spring media for perpendicular recording,” AppliedPhysics Letters, vol. 87, 30. June 2005. pp. 012504, by Suess et al.,incorporated herein by reference in its entirety, domain wall assistedrecording on bilayers was presented. Subsequent work by A. Dobin and H.J. Richter (presented at the Intermag conference 2006, talk DB-10, SanDiego, Calif., May 2006; preprint available at http://arxiv.org “DomainWall Assisted Magnetic Recording” by Dobin and Richter) followed thesame approach.

In previous works it was not shown that a finite value of the anisotropyin the soft magnetic layer does not reduce the thermal stability of thestructure. This question needs to be investigated, as largeranisotropies in the softer layer reduce the energy that is required topush a domain wall from the soft layer to the hard layer. Further,typical multilayer exchange spring media do not even contain a softmagnetic layer. Instead, it contains a nucleation layer which can bemagnetically hard. The softest layer in the nucleation layer can have acoercive field similar to typical fields of recording heads.

For all these reasons, the choice of layer architectures and theiranisotropies to overcome the superparamagnetic limit in optimal fashionremain a topic of intense investigations.

SUMMARY

Briefly and generally, embodiments according to the invention include amagnetic recording media wherein the magnetic recording layer consistsof a multilayer structure, with a special multilayer host layer(nucleation host) and a hard magnetic storage layer (He>2T). Theanisotropy of the softest layer in the nucleation host is significantlysmaller than in the hard layer. In some embodiments the ratio ofanisotropies can reach a factor of 2. During writing a domain wall isformed in the nucleation host that propagates through the whole grainstructure and finally reverses the hard magnetic storage layer. Thenucleation host significantly decreases the coercive field of each grainof the proposed media but has only little influence on the thermalstability. In some embodiments the host layer comprises of just onelayer, the media is a bilayer structure with one hard magneticnucleation host and an even harder storage layer. In embodiments, wherethe host layer comprises more than one layer the anisotropy increasesfrom one layer to the next layer. The layers can be strongly exchangecoupled. The exchange coupling can be direct or via a thin couplinglayer in order to achieve strong coupling. If the layers are stronglycoupled a domain wall is formed across the hard/soft interface duringreversal.

In embodiments with a nucleation host having the same magnetization asthe hard magnetic storage layer and consisting only of one layer, thecoercive field can be reduced by a factor of up to five compared to thecoercive field of the hard magnetic storage layer without the nucleationhost at the same thermal stability.

In embodiments, where the nucleation host has an exchange constant andmagnetic polarization in the nucleation host larger than in the hardmagnetic storage layer, the reduction of the coercive field can be evenlarger than a factor of five.

The coercive field can also be increased by increasing the number oflayers in the nucleation host. In embodiments with two or three layersin the nucleation host, the coercive field is reduced by a factor of upto 9 and 13, compared to the coercive field of the hard magnetic storagelayer without the nucleation host.

In other embodiments further increase of the coercive field is reachedby increasing the anisotropy continuously. For a 25 nm thick layer withquadratically increasing anisotropy and a maximum value of K₁=2 MJ/m³ inthe nucleation host the coercive field is smaller by a factor of 10compared to a layer consisting of a material with K₁=2 MJ/m³, while,remarkably, the thermal stability of these two architectures issubstantially the same.

Embodiments also show a characteristic dependence of the coercive fieldon the angle between the external field and the easy axis of the hardlayer. In contrast to single phase recording media that shows aStoner-Wohlfarth like switching field dependence as a function of theexternal field angle, exchange spring media behave more like “pinningmagnets”. In pinning permanent magnets a high coercive field can beachieved by the introduction of pinning centres, such as soft magneticinclusions or non magnetic inclusions that trap a domain wall and hencehinder the domain wall from propagating through the whole magnet.Therefore, the reversal of the magnetization due to domain wall motionis stopped by the pinning centres. Typically, in pinning magnets thedependence of the coercive field as a function of the angle α betweenthe external field and the easy axis can be described by,H_(c)=1/cos(α). That implies that for small angles between the externalfield and the easy axis the switching field is less sensitive to theeasy axis distribution. This makes exchange spring media also apotential candidate for patterned media since the switching fielddistribution due to an easy axis distribution is reduced.

Generally, the above embodiments provide marked improvement against thesuperparamagnetic limit as the coercive field is reduced by thespatially varying anisotropy in the nucleation host, while the thermalstability is determined only by the domain wall energy in the hardestmagnetic storage layer, which is not influenced by the variations of theanisotropy of the nucleation host.

This is achieved because the new architecture removes theproportionality of the energy barrier to the coercive field. In somecases the coercive field can be inversely proportional to the layerthickness while the energy barrier remains independent of it. Therefore,with these new architectures embodiments overcome the writeabilityproblem of extremely hard magnetic storage layers with high thermalstability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of a magnetic recording disk with amultilayer exchange spring recording media.

FIG. 2 shows a hysteresis loop in some embodiments. For comparison, thehysteresis loop of a conventional single phase grain is shown.

FIG. 3 shows the coercive field as a function of the angle between theeasy axis and external field for bilayers with different values of theanistropy in the softest layer in the nucleation host.

FIG. 4 shows the hysteresis loop of different exchange spring filmscomprising of 14×14 grains.

FIG. 5 illustrates the reduction of the coercive field for a nucleationhost with a continuously increasing anisotropy.

FIG. 6 shows the energy barrier as a function of the hard layerthickness.

DETAILED DESCRIPTION

FIG. 1 illustrates the cross sectional view of the layer structure of anexisting disk of recording medium. The disk includes a substrate 5, asoft magnetic underlayer 7, an interlayer 9, an optional seed layer 11 aand underlayer 11 b, a hard magnetic storage layer 24, strongly exchangecoupled to a nucleation layer 21, an optional coupling layer 22, aprotective layer 28 a, and a lubricant layer 28 b. Although the hardmagnetic storage layer is shown on top of the nucleation layer, theorder of the layers can also be reversed.

Substrate 5 may be made of any suitable material such as ceramic glass,amorphous glass, or NiP plated AlMg or an AlMg alloy base with a NiPsurface coating.

Soft magnetic underlayer 7 is deposited on substrate 5, soft magneticunderlayer 7 may be made of any suitable material such as NiFe, CoNbB,FeAlSi, CoFeB, FeTaN, FeTaC, FeCoB, FeSiO, FeAlSi, FeTaN, FeN, CoFe,CoZr, CoFeB, CoZrNb, NiFeNb or equivalents. Soft magnetic underlayer 7may have a thickness in the range of about 50 nm to about 500 nm.

Interlayer 9 maybe important for the grain structure of the hardmagnetic recording media. Interlayer 9 maybe any suitable material suchas Pt, Ge, Si, CoCr, SiO₂, Au, Al, CoZr, Ta, Ti, TiCr, Ru, RuCrCo, TiZr,or equivalents.

Optional seed layer 11 a maybe used in order to improve the growth ofunderlayer 11 b. Optional seed layer 11 a may be formed of a suitablematerial of hcp, fcc, bcc or even amorphous structure. It provides asmooth wetted surface for the subsequent growth of underlayer 11 b.

Underlayer 11 b is deposited onto optional seed layer 11 a, if present,or otherwise directly onto substrate 5. Underlayer 11 b may be formed ofa suitable hcp material with a lattice parameter close to the alloys ofthe first layer in the magnetic layer. These alloys may be, for example.Ru-based alloys, Re-based alloys or Pd based alloys. Underlayer 11 b mayhave a thickness in the range of about 1 nm to about 20 nm. Otherthicknesses can also be used.

The recording media includes hard magnetic storage layer 24 and softernucleation host 21. Hard magnetic storage layer 24 can be a ferromagnet,ferrimagnet, antiferromagnet, or any other synthetic antiferromagneticstructure. The coupling between the nucleation host and the hardmagnetic storage layer may be ferromagnetic or antiferromagnetic. Hardmagnetic storage layer 24 maybe formed from any material that has largeperpendicular anisotropy. These materials include tetragonal:L1₀-ordered phase materials, CoPt and FePt based alloys, CoPtCr alloys,including CoPtCrB, CoPtCrTa, and CoCr based granular media. Other highanisotropy materials suitable for the recording layer 16 includepseudo-binary alloys based on the FePt and CoPt L1₀ phase. i.e., FePt—Xand CoPt—X, where the element X may be Ni, Au, Cu, Pd or Ag, as well asgranular composite materials such as FePt—C, FePt—ZrO, FePt—MgO,FePt—B₂O₃, materials containing at least one of B, Cu, Ag, W, Mo, Ru,Si, Ge, Nb, Pd, Sm, Nd, Dy, Hf, Mn, Ni and other similar composites.

In some embodiments the thickness of hard magnetic storage layer 24 isbetween 3 nm and 30 nm. In other architectures it can be outside theseranges. In embodiments where the anisotropy of nucleation host 21 iscontinuously increased, hard magnetic storage layer 24 is optional.

In some embodiments nucleation host 21 can be formed on hard magneticstorage layer 24. In other embodiments the order is changed so that hardmagnetic storage layer 24 is deposited on nucleation host 24. Nucleationhost can be formed as a granular or a continuous film. The exchangecoupling between hard magnetic storage layer 24 and nucleation host 21can be sufficiently strong to enable the formation of a domain wallacross the interface of these layers during reversal.

Any of the materials, listed for hard magnetic storage layer 24 can bealso used for forming nucleation host 21. In some embodiments a magneticmaterial is referred to as “hard” if its coercive field Hc>0.5 T, and“soft” is its coercive field is Hc<0.5 T. In some embodiments theseterms are defined in a relative sense; the coercive field of (soft)nucleation host 21 can be half of the coercive field of hard magneticstorage layer 24. In other embodiments this ratio can be different. Insome embodiments a layer is referred to as “hard”, ifΔk_(max)/k_(min)<0.5. The definition of the terms in this inequalitywill be given below.

In some embodiments the grains of nucleation host 21 are aligned withthe grains of hard magnetic storage layer 24, as shown in FIG. 1. Insome embodiments this alignment is only approximate. These embodimentsare sometimes called columnar media.

In some embodiments nucleation host 21 and hard magnetic storage layer24 are in direct contact. In other embodiments, they are separated byoptional coupling layer 22. Optional coupling layer 22, disposed betweenhard magnetic storage layer 24 and nucleation host 21, can provide astrong “exchange coupling”. Optional coupling layer 22 may enhance thegranular growth between hard magnetic storage layer 24 and nucleationhost 21. Optional coupling layer 22 may have a thickness between 0.1 nmand 3 nm. Optional coupling layer 22 may provide an exchange constant Ain excess of A=10⁻¹⁴ J/m. In conventional perpendicular recon dingheexchange constant between grains is small. In exchange spring media bothlarge and small values of the lateral exchange can be useful. For alarge value of the lateral exchange in hard magnetic storage layer 24the transitions of the bits are not necessarily located at the grainboundaries. Instead, domain, walls are formed that separate the bits. Alarge value of the anisotropy of hard magnetic storage layer 24decreases the domain wall width, so that the transition parameter of thebits may not be significantly larger than 2 nm to 3 nm that can be equalto about the grain diameter. In the case of large lateral exchange thedomain walls are not pinned at the grain boundary in the hard magneticstorage layer but the domain walls are pinned due to the granularlaterally weakly exchange coupled nucleation host. A weak exchange inthe hard magnetic storage layer may require a large exchange in thenucleation layer.

In the following the plane of nucleation host 21 will be identified asthe x-y plane. For example, in a magnetic storage disk, the plane of thedisk is essentially parallel to the x-y plane of nucleation host 21. Thez coordinate then parametrizes the direction of the thickness ofnucleation host 21.

In some embodiments nucleation host 21 contains more than one layerwhich have different anisotropy K. In other embodiments nucleation host21 is characterized by a spatially varying anisotropy K(z). In theseembodiments, the anisotropy assumes more than one value in a substantialmagnetic portion of nucleation host 21. In this sense the embodimentsdiffer from single layer nucleation hosts, whose anisotropy assumesvarying values only in an insubstantial portion. This may occur e.g.because of surface effects, in a very thin layer at the surface. Also,the variation is occurring in the magnetic nucleation host 21 itself. Inthis sense, embodiments differ from disks which have a single layernucleation host 21 and a spacer layer.

In some embodiments, the softest, or smallest value of the anisotropy Kis half of the anisotropy of hard magnetic storage layer 24. In otherembodiments this ratio can be closer to one. This softest value of K isthe anisotropy of the softest layer in multilayer embodiments, or thesoftest value in the spatially varying embodiments.

In some embodiments the lateral, or nearest neighbor, exchange betweenthe grains of nucleation host 21 is small.

Protective layer 28 a, sometimes called overcoat, is typicallydiamond-like amorphous carbon or nitrogenated carbon, but may be anyconventional disk overcoat. Overcoat 28 a is typically less than 15 nmthick.

All of the layers described above from seed layer 11 to overcoat 28 acan be sputtered in a continuous process in either an in-line sputteringsystem or a single disk system, such as commercially available singledisk systems with multiple sputtering target capacity. The sputterdeposition of each of the layers can be accomplished using standardtargets and techniques known to those in the field with themodifications described above.

In the following, some considerations will be provided, which may berelevant for understanding features of the embodiments. Some embodimentscombine benefits of softer magnetic layer with that of harder magneticlayers.

In some embodiments nucleation host 21 includes only one layer. In theseembodiments the recording media is referred to as a bilayer, whichincludes nucleation host 21 and hard magnetic storage layer 24. Underthe assumption that both layers remain completely homogeneous, thebilayer structure does not provide any improvements for the writeabilitycompared to a single layer system. This conclusion is different fromthat of Y. Inaba et al. in the paper “Preliminary Study on(CoPtCr/NiFe)—SiO2 Hard/Soft-Stacked Perpendicular Recording Media”,IEEE Transactions on Magnetics, Vol. 41. No. 10, October 2005, pp. 3136.

A hard/soft layer structure where the magnetization in the hard and softlayer is parallel and uniform can be described with one averagemagnetizationM_(eff)=(M_(hard)×I_(hard)+M_(soft)×I_(soft))/(I_(soft)+I_(hard)) andone average anisotropy constantK_(eff)=(K_(hard)×I_(hard)+K_(soft)×I_(soft))/(I_(soft)+I_(hard)). In asimple model the energy barrier can be estimated byΔE=F×K_(eff)×(I_(soft)+I_(hard)), where F is the area of the basal planeof one grain of the media. The coercive field is H_(c)=2K_(eff)/M_(eff). The ratio of the energy barrier to the coercive field,the “ratio barrier” ΔE/He only depends on the value of the averagemagnetization and the grain volume. Therefore, this bilayer structureleads to the same thermal stability as the single layer structure of thesame thickness for the same value of the coercive field and for the samevalue of the average magnetization.

In embodiments of the invention, the design of the recording media iscapable of supporting the formation of a domain wall during reversal.This magnetic domain wall becomes pinned at hard/soft interfaces.Compared to single phase media with the magnetic properties of the hardlayer, the coercive field in a bilayer structure can be significantlyreduced. In embodiment this reduction reaches a factor of four, asdescribed by Loxley et al. in the paper: “Theory of Domain WallNucleation in a Two Section Magnetic Wire”, IEEE Transactions onMagnetics. Vol. 37, July 2001, pp. 1998-2100). However, Loxley et alconsidered an idealized magnetic wire instead of a magnetic recordingmedium. A factor of five decrease was shown by Hagedorn in the paper:“Analysis of Exchange-Coupled Magnetic Thin Films,” Journal of AppliedPhysics, Vol. 41, 1970, pp. 2491-2502, when a finite anistropy in thesoft layer was assumed.

A feature of embodiments of the invention is that the coercive field andthe thermal energy barrier can be separately adjusted. In contrast, theabove works considered these quantities closely connected and thus notadjustable separately. The thermal energy barrier is primarilydetermined by the material parameters of hard magnetic storage layer 24,substantially independently from the material parameters of nucleationhost 21. Thus, in embodiments the ratio

$r = \frac{{\Delta E}_{thermal}}{{\Delta E}_{hyst}}$

can be selected in the range of 0.5 to 10. In others the range can beeven broader. Here the thermal energy barrier ΔE_(thermal) is the energybarrier separating the two magnetic states, which has to be overcome bythe thermal fluctuations. The hysteretic energy lossΔE_(hyst)=J_(s)H_(c)V is the product of grain volume V, the coercivefield H_(c) and the average magnetization J_(s) which characterizes theenergy to be overcome during a write process. For single domainparticles these energy scales are closely connected with a fixed ratio rof 0.5.

The field, required to overcome the pinning field to push a domain wallfrom the softer layer to the hard layer depends on the differencebetween the anistropy constants of these layers as described by aformula by Hagedorn et al. If the number of layers is increased, thisdifference can be decreased leading to a reduction of the pinning andcoercive field. For example, if the anisotropy of the m-th layer assumesthe value K^(m)=(m−1)K_(hard)/(N−1), the difference between theanisotropy constants of adjacent layers is K_(hard)/(N−1). Such anarchitecture leads to a coercive field of the whole structure ofH_(c)=1/(4N−4)×2 K_(hard)/J_(s). Here. K_(hard) is the anisotropyconstant of the hardest layer (the hard magnetic storage layer), J_(s)the magnetic polarization of the structure and N is the number oflayers. For sufficiently thick hard magnetic storage layer the thermalstability is given by the domain wall energy in the hard magneticstorage layer. Importantly, therefore, at zero field the nucleation hostdoes not lower the thermal stability.

FI. 2 illustrates the micromagnetic simulations, which were performed todemonstrate the benefit of multilayer exchange spring media. FIG. 2compares the hysteresis loop of different multilayer exchange springmedia with a conventional recording media. Only one grain of the mediawas modeled. The parameters are for illustrative purposes only and canwidely vary in different embodiments. The external field is applied atan angle of 0.5°. Nucleation host 21 is assumed to have a granularstructure. The thickness of the entire grain structure is 25 nm. Thegrain diameter is 5 nm. The magnetization is M_(s)=0.5 T/μ₀. The grainsare formed from a magnetic hard material with K₁=2 MJ/m³.

The trace “single phase” illustrates an embodiment in which the grainshave a single anisotropy value. The trace “bilayer soft K_(soft)=0”illustrates an embodiment, where 7 nm of the grain are magneticallyextremely hard with K_(hard)=2 MJ/m³, the other 18 nm are perfectly softK_(soft)=0. The trace “bilayer soft K_(soft)=0.2 MJ/m³” illustrates anembodiment where again 7 nm of the grain is extremely hard K_(hard)=2MJ/m³. The softer region has still a relative high anisotropy ofK_(soft)=0.2 MJ/m³. The “trilayer” trace illustrates the embodimentwhere 7 nm again has K⁻¹=K_(hard)=2 MJ/m³, 5.6 nm of the grain hasK²=1.11 MJ/m³ and 12.4 nm of the grain has K⁻³=0.22 MJ/m³.

Visibly, the coercive field of the bilayer structures is severelyreduced from that of the single phase media by a factor of about 4-4.5.The trilayer structure has its coercive field further reduced, by abouta factor of 7 compared to the single phase media. The discrepancy to thetheoretical limit of 9 can be attributed to the finite layer thickness.The effect of reversible magnetization processes can be observed by thenon-rectangular shape of the hysteresis loop.

Some embodiments have a gradually varying anisotropy K(z) in nucleationhost 21. Such embodiments will be referred to as “G-layer”. In FIG. 2the hysteresis loop of G-layers are shown. The trace “continuousstrayfield” illustrates the embodiment in which the anistropy increasesquadratically as a function of the z coordinate, the distance from thebottom of the grain; K(z)˜z². The demagnetizing field of the grain istaken into account. The trace “continuous” illustrates the embodiment inwhich the demagnetizing field was omitted. FIG. 2 shows that thecoercive field is just 1/10 of the coercive field of the single layerstructure for a thickness of 25 nm. It is also interesting to note thatthe hysteresis loop is almost perfectly rectangular. If the strayfieldis taken into account, the coercive field increases by about 3%, whichcan be attributed to the additional uniaxial anisotropy caused by theshape anisotropy.

The variation of K(z) can take many different form. The rise can belinear, or follow any polynomial or other rising function. It can haveplateaus or steps, connected by rising segments. It can also havedecreasing segments. In some embodiments, the functional form of K(z) ischosen to optimize the switching properties of the recording media, e.g.by increasing the thermal energy barrier while keeping the writingfield, or coercive field, essentially unchanged, or even decreasing it.

FIG. 3 illustrates the angular dependence of the coercive field H_(c)(α)as a function of the angle α that is measured between the easy axis andthe external field. So the coercive field is measured for differentangles α. The solid line shows the angular dependence of the trilayer ofFIG. 2. The dotted line shows the angular dependence of the coercivefield if the anisotropy in the softest layer of the nucleation host isset to zero. In contrast to single phase media, the coercive field doesnot have a minimum for a field angle of 45°. The pinning force to depinthe domain wall at the hard/soft interfaces depends inversely from thecosine of the angle between the easy axis and the external field. It canbe seen that the coercive field H_(c)(α=20°) is smaller thanH_(c)(α=45°). A similar dependence can be found if instead of thecoercive field the field H_(m)(α) is introduced, where H_(m) is thefield that is required to decrease the magnetization of the saturatedfilm (M_(z)/M_(s)=1) to M_(z)/M_(s)=−0.5. Differences of the field H_(m)and H_(c) occur if the media comprises of a thick soft layer with highmagnetization.

FIG. 4 shows the hysteresis loop of an exchange spring perpendicularrecording film consisting of 14×14 grains. The parameters of thisembodiment are for illustrative purposes only. The average graindiameter is 5 nm. The film thickness is 23 nm. The exchange fieldbetween the grains is about 0.25 T. The thickness of the nucleationlayer is equal to the thickness of the hard magnetic storage layer whichis 10.5 nm. The two layers are separated by a 2 nm thick optionalcoupling layer 22. The exchange constant in the coupling layer is variedfrom A=1×10⁻¹² J/m to the bulk value of the exchange in the nucleationlayer and the storage layer which is A=1×10⁻¹¹ J/m. The hysteresis loopsshow that with increasing coupling the coercive field decreases.Furthermore the hysteresis curves for A=1×10⁻¹² J/m and A=2×10⁻¹² J/mshow that the slope of the hysteresis loop

$\mspace{20mu}{k = {\frac{dM}{dH}\frac{H_{\text{?}}}{H_{\text{?}}}}}$?indicates text missing or illegible when filed

changes in the interval −0.7<M/M_(s)<0.7, significantly. For A=2×10⁻¹²J/m the slope k changes in the interval −0.7<M/M_(s)<0.7 by about afactor of 2.5. In embodiments the anisotropy of hard magnetic storagelayer 24 and the exchange coupling between nucleation host 21 and hardmagnetic storage layer 24 is chosen to keep the change of the slope ksmaller than 3. A ratio k_(max)/k_(min)<3 indicates a strong couplingbetween nucleation host 21 and hard magnetic storage layer 24.

Furthermore the squareness S of the hysteresis loops was calculated.

$\mspace{20mu}{S = \left. {1 - {\frac{dM}{dH}\frac{H_{\text{?}}}{M_{\text{?}}}}} \middle| {}_{H_{\text{?}}}{{.\text{?}}\text{indicates text missing or illegible when filed}} \right.}$

For the fully coupled case (A=1×10⁻¹¹ J/m) the squareness is 0.8. It isimportant to note that for all calculations the hysteresis loops are notdesheared.

FIG. 5 illustrates the reduction of the coercive field for nucleationhost 21 with thickness t_(G) and a continuously increasing anisotropy,fully exchange coupled to the hard magnetic storage layer. The value ofthe anisotropy constant of the hard magnetic storage layer is changedfrom 1 MJ/m³ to 4 MJ/m³. FIG. 4 shows the reduction of the coercivefield of hard magnetic storage layer 24 with a thickness of 20 nm and avalue of the anisotropy constant of K_(hard) that is coupled to aG-layer. The anisotropy constant depends quadratically on the depth z inthe layer, K(z)=z²K_(hard)/t_(G) ². The grain has the maximum anisotropy(K_(hard)) at the surface of nucleation host 21. The thickness ofnucleation host 21 is t_(G). This embodiment does not have hard magneticstorage layer 24.

FIG. 6 illustrates the influence of the thickness of hard magneticstorage layer 24 on the energy barrier of the bilayer structure. Theparticular parameters are for illustrative purposes only—otherembodiments have different parameters. The thickness of nucleation hostis 20 nm (limit of infinite thick soft layer). The anisotropy in hardmagnetic storage layer 24 is K_(hard)=1×10⁶ J/m³. The grain diameter is6 nm. The energy barrier levels out for thicknesses larger than 20 nm.It is important to stress that at zero field the energy barrier does notdepend on nucleation host 21. Due to thermal fluctuations a domain wallis formed in the nucleation host. The energy of the domain wallsuccessively increases as it is driven by thermal fluctuations in theharder layers. The domain wall has largest energy when it is located inthe hardest layer. The difference this highest energy state minus theenergy of the homogenous state when the spins in the grain point up(down) denotes the energy barrier. For all structures the highest energystate is given by the domain wall energy in the hard magnetic storagelayer which is independent if a nucleation host 21 is present. Thedemagnetizing field of the structure may slightly decrease the energybarrier. The decrease of the energy barrier may be larger if anucleation host is present as without nucleation host 21. In embodimentsthe energy barrier of a typical grain of hard magnetic storage layer 24is smaller by 25% of the energy barrier of a typical grain of hardmagnetic storage layer 24 without the nucleation host at most.

We note that parameters of an unknown magnetic recording media can becharacterized by different methods. For example, an energy barrier of ahard magnetic storage layer can be determined: (i) by depositing onlythe hard magnetic storage layer on a non-magnetic substrate, or (ii) byremoving a nucleation host from a multilayer structure- or (iii) byperforming a micromagnetic calculation with parameters appropriate forthe multilayer.

While the present invention has been particularly shown and describedwith reference to certain embodiments, it will be understood by thoseskilled in the art that various changes in form and detail may be madewithout departing from the spirit and scope of the invention.Accordingly, the disclosure was intended merely as illustrative and thescope of the invention is limited only as specified in the appendedclaims.

1. A magnetic recording system, comprising: a writing head; and a disk,including a magnetic recording medium, comprising an essentiallynon-magnetic substrate; and a magnetic bilayer, the magnetic bilayerincluding a granular hard magnetic storage layer with perpendicularanisotropy having a coercive field of H, without another magnetic layerand a thickness between 3 nm and 30 nm; and a granular hard magneticnucleation host with perpendicular anisotropy, disposed on the hardmagnetic storage layer in a columnar manner, having a coercive fieldH_(a) without the hard magnetic storage layer, wherein 0.5T<H_(n)<H_(s); the nucleation host has a magnetic polarizationJ_(s)=μ₀M_(s) between 0.3 T and 1.0 T; the nucleation host and the hardmagnetic storage layer are separated by a coupling layer between 0.1 nmand 3 nm thickness; the coupling layer is directly between thenucleation host and the storage layer; and the coupling between thenucleation host and the hard magnetic storage layer is ferromagnetic. 2.The magnetic recording system of claim 1, wherein: the coupling layerprovides an exchange constant A>10⁻¹⁴ J/m between the nucleation hostand the hard magnetic storage layer.
 3. The magnetic recording system ofclaim 1, wherein: a squareness S of a hysteresis loop of the magneticrecording medium is less than 10% different from a squareness of ahysteresis loop of the hard magnetic storage layer without thenucleation host.
 4. The magnetic recording system of claim 1, whereinthe nucleation host comprises: at least one of Fe, and O.
 5. Themagnetic recording system of claim 1, wherein the nucleation hostcomprises: grains with an average diameter greater than 2 nm and lessthan 10 nm.
 6. The magnetic recording system of claim 1, wherein: athickness of the nucleation host is larger than 7 nm.
 7. The magneticrecording system of claim 1, wherein: the hard magnetic storage layer bformed based on a FePt alloy.
 8. The magnetic recording system of claim1, wherein: the hard magnetic storage layer being formed of an alloyselected from the group consisting of CoPt alloys, CoPtCr alloys,CoPtCrB alloys, CoPtCrTa alloys, and CoCr alloys.
 9. A magneticrecording system, comprising: a writing head; and a disk, including amagnetic recording medium, comprising an essentially non-magneticsubstrate; and a magnetic bilayer, the magnetic bilayer comprising: agranular hard magnetic storage layer having a coercive field of H_(s)without another magnetic layer; and a granular hard magnetic nucleationhost, disposed on the hard magnetic storage layer in a columnar manner,having a coercive field H_(n) without the hard magnetic storage layer,wherein 0.5 T<H_(n)<H_(s); and H_(c)(α=20°)<H_(c)(α=45°), whereinH_(c)(α) is a coercive field of the recording medium if an angle betweenan external magnetic field and a normal of the magnetic bilayer is α,wherein the nucleation host and the hard magnetic storage layer arearranged as one of in direct contact and separated by a coupling layerof less than 5 nm thickness.
 10. The magnetic recording system of claim9, wherein: a squareness S of a hysteresis loop of the magneticrecording medium is less than 10% different from a squareness of ahysteresis loop of the hard magnetic storage layer without thenucleation host.
 11. The magnetic recording system of claim 9, whereinthe nucleation host comprises: grains with an average diameter greaterthan 2 nm and less than 10 nm.
 12. The magnetic recording system ofclaim 9, wherein: a thickness of the nucleation host is larger than 7nm.
 13. The magnetic recording system of claim 9, wherein: thenucleation host is coupled to the hard magnetic storage layer by thecoupling layer, providing a ferromagnetic coupling between thenucleation host and the storage layer.
 14. The magnetic recording systemof claim 9, wherein the nucleation host comprises: at least one of Fe,and O.
 15. The magnetic recording system of claim 9, wherein: the hardmagnetic storage layer being formed based on a FePt alloy.
 16. Amagnetic recording medium, comprising: an essentially non-magneticsubstrate; and a magnetic bilayer, the magnetic bilayer including agranular hard magnetic storage layer, based on L1₀-phase FePt, with aperpendicular anisotropy having a coercive field of H_(s) withoutanother magnetic layer and a thickness between 3 nm and 30 nm; and agranular hard magnetic nucleation host with perpendicular anisotropy,disposed on the hard magnetic storage layer in a columnar manner, havinga coercive field H_(n) without the hard magnetic storage layer, whereinH_(n)<H_(s); and the nucleation host has a magnetic polarizationJ_(s)=μ₀M_(s) between 0.3 T and 1.0 T.
 17. The magnetic recording mediumof claim 16, wherein: the nucleation host and the hard magnetic storagelayer are separated by a coupling layer between 0.1 nm and 3 nmthickness, the coupling layer is directly between the nucleation hostand the storage layer; and the coupling between the nucleation host andthe hard magnetic storage layer is ferromagnetic.
 18. The magneticrecording medium of claim 16, wherein: the hard magnetic storage layerincludes an alloy based on FePt—X, wherein X is one of Ni, Au, Cu, Pdand Ag.
 19. The magnetic recording medium of claim 16, wherein: the hardmagnetic storage layer includes a granular composite material based onFePt—C.
 20. The magnetic recording medium of claim 16, wherein: the hardmagnetic storage layer includes a granular composite material, selectedfrom the group consisting of FePt—ZrO, FePt—MgO, and FePt—B₂—O₃.
 21. Themagnetic recording medium of claim 16, wherein: the hard magneticstorage layer includes materials, selected from the group consisting ofB, Cu, Ag, W, Mo, Ru, Si, Ge, Nb, Pd, Sm, Nd, Dy, Hf, Mn, Ni.
 22. Amagnetic recording medium, comprising: an essentially non-magneticsubstrate; and a magnetic bilayer, the magnetic bilayer comprising: agranular hard magnetic storage layer, based on L1₀-phase FePt, having acoercive field of H_(s) without another magnetic layer; and a granularhard magnetic nucleation host, disposed on the hard magnetic storagelayer in a columnar manner, having a coercive field H_(n) without thehard magnetic storage layer, wherein H_(n)<H_(s); andH_(c)(α=20°)<H_(c)(α=45°), wherein H_(c)(α) is a coercive field of therecording medium if an angle between an external magnetic field and anormal of the magnetic bilayer is α, wherein the nucleation host and thehard magnetic storage layer are arranged as one of indirect contact andseparated by a coupling layer of less than 5 nm thickness.
 23. Themagnetic recording medium of claim 22, wherein: the granular hardmagnetic storage layer includes an alloy based on FePt—X, wherein X isone of Ni, Au, Cu, Pd and Ag.
 24. The magnetic recording medium of claim22, wherein: the granular hard magnetic storage layer includes agranular composite material based on FePt—C.
 25. The magnetic recordingmedium of claim 22, wherein: the granular hard magnetic storage layerincludes a granular composite material, selected from the groupconsisting of FePt—ZrO, FePt—MgO, and FePt—B₂O₃.
 26. The magneticrecording medium of claim 22, wherein: the granular hard magneticstorage layer includes materials, selected from the group consisting ofB, Cu, Ag, W, Mo, Ru, Si, Ge, Nb, Pd, Sm, Nd, Dy, Hf, Mn, Ni.